An ion cyclotron uses a magnetic field to deflect an ion moving at some velocity through the field. For a spatially uniform magnetic field having a flux density B, a moving ion of mass m and charge q will be bent into a circular path in a plane perpendicular to the magnetic field at an angular frequency .omega..sub.0 in accordance with .omega..sub.0 =qB/m. Thus, if the magnetic field strength is known, by measuring the ion cyclotron frequency it is possible in principle to determine the ionic mass-to-charge ratio m/q. In effect, the static magnetic field converts ionic mass into a frequency analog. Because the cyclotron frequencies for singly charged ions (12.ltoreq.m/q.ltoreq.5,000) in a magnetic field of about 3 Tesla span a radio-frequency range (10 KHz.ltoreq.f.ltoreq.4 MHz) within which frequency can be measured with high precision, the ion cyclotron is potentially capable of offering extremely high mass resolution and accuracy.
Fourier transform techniques have been utilized in the detection scheme of ion resonance in mass spectrometry. In such techniques, the whole spectrum of ions is excited at once and the whole spectrum is thereafter detected at once. Such Fourier transform ion cyclotron resonance spectroscopy techniques are described further in U.S. Pat. No. 3,937,955 to Comisarow, et al., the disclosure of which is incorporated herein by reference. The Comisarow, et al. patent describes a so-called sweep or chirp excitation in which the excitation sinusoid is swept from one frequency to another to excite all ions whose cyclotron frequencies are in that range. Because this is a frequency modulated signal, the shape of its amplitude spectrum is not available as a convenient closed-form equation. The spectral shape is generally a single band with relatively uniform amplitude at the band's center, amplitude ripples which are worse at the band edges, and a gradual decrease in ripple amplitude towards zero outside the band. Both the intensity and location of the ripples as well as the sharpness of the band edges depend on the sweep parameters (sweep rate, start and stop frequencies) in such a manner that arbitrarily sharp band edges and a low ripple cannot be achieved at the same time. In addition, sweep excitation necessarily excites all ions with resonant frequencies between the sweep start and stop frequencies (with broad band sweep) and thereby does not allow selective excitation of ions with only certain ranges of mass-to-charge ratios (hereafter denoted m/z). Such broad band excitations also cannot be used to eject ions selectively of all but one or a few selected m/z values.
A simpler excitation technique is a burst excite, in which a fixed frequency, fixed amplitude sinusoidal signal is applied to the cell excitation plate for a fixed time. This excitation signal has a (sin x)/x shape (a sinc function) in its frequency domain magnitude spectrum. It is possible, by using burst excite, to excite ions of one m/z to a desired orbital radius while not exciting at all ions of a second m/z. However, the only adjustable parameters are the sinusoidal frequency, amplitude, and duration, so that the excite amplitude spectrum can only have a (sin x)/x shape, which is not suitable when ions of many m/z are present.
Another ion excitation method for Fourier transform ion cyclotron mass spectrometry is based on sinusoidal bursts and may be denoted pulse sequence excitation. The sequence of sinusoidal bursts is constructed with the frequency, phase and starting time of each burst selected such that the amplitude spectrum of the sequence approximates the desired excite amplitude spectrum. High selectivity is possible for simple spectral shapes, but it is difficult to construct pulse sequences to approximate arbitrary excite spectra.
Impulse excitation consists of a single narrow pulse. This method is broadband only, so that no selectivity is possible. Also, very high voltages are required to deliver sufficient energy to the ions due to the short duration of the pulse.
Pseudo-random noise excitation uses a white noise sequence to excite ions over a wide mass range. No selectivity is possible with this method either, but much lower voltages are required than for impulse excitation.
An improved technique for tailoring the excite amplitude spectrum to excite ions of particular m/z values is set forth in U.S. Pat. No. 4,761,545 to Marshall, et al., the disclosure of which is incorporated herein by reference. In the method of that patent, which may be denoted as stored waveform inverse Fourier transform excitation, an arbitrary selected excitation amplitude spectrum is inverse Fourier transformed to give a time domain waveform. That waveform is then used as the excitation signal. One difficulty with the procedure is that the resulting time domain waveform can have a very high peak to average power ratio, particularly when the excite amplitude spectrum is broadband. Another difficulty is that if there exist any discontinuities in the starting excite spectrum or in any order derivative of this spectrum, truncating the resulting time domain waveform to finite length introduces Gibbs oscillations into the corresponding excite amplitude spectrum. A window function having a value of zero at both ends can remove the Gibbs oscillation but can cause distortion of a stored waveform which has been phase scrambled. An extension of the stored waveform excitation technique is described in U.S. Pat. No. 4,945,234 to Goodman, et al., the disclosure of which is incorporated herein by reference. That patent provides a technique for providing an excitation signal starting with a frequency domain spectrum and operating on it in such a manner to reduce the Gibbs oscillations of the excitation signal. While such techniques are useful and have been utilized to provide highly tailored excitation spectra, considerable computer processing is required to create the ultimate time domain excitation signal from the initial frequency domain spectrum.